1. Field of the Invention
The present invention generally relates to an infrared transparent optical element, particularly the optical element transparent to infrared rays of light within a wavelength range of about 8 to 12 xcexcm and an infrared imaging camera using such infrared transparent optical element as a window.
2. Description of the Related Art
An infrared imaging camera of a kind generally used in severe environments makes use of an infrared transparent window to isolate the remainder of the imaging system from exposure to humidity, corrosive and abrasive environments. The infrared transparent window has, deposited on a surface thereof, a protective film of an infrared transparent material capable of transmitting therethrough infrared rays of light within the wavelength region of about 8 to 12 xcexcm. The Japanese Laid-open Patent Publications No. 1-259301 (corresponding to U.S. Pat. No. 4,907,846 issued Mar. 13, 1990) and No. 64-56401 published Oct. 17, 1989 and Mar. 3, 1989, respectively, disclose a diamond layer and a carbon layer having a diamond structure (i.e., a diamond-like carbon layer), used for a protective film on the infrared transparent optical element.
FIG. 6A illustrates a transverse sectional representation of the prior art infrared transparent window 30 hitherto used in the conventional infrared imaging camera, and FIG. 6B is a plan view of the infrared transparent window 30 of FIG. 6A as viewed in a direction conforming to the direction of incidence of light indicated by the arrow in FIG. 6A. The illustrated window 30 is made up of an infrared transparent substrate 32 made of, for example, germanium, an antireflection film 34 deposited on each of opposite surfaces of the window 30. The protective film referred to above is indicated by 36 and is formed on one of the antireflection films 34 which faces towards the outside of the infrared imaging camera.
Other than germanium, any of inorganic monocrystalline materials such as silicon, ZnS, ZnSe, KBr, KCI, NaCl, CsI and CsBr can be used as a material for the infrared transparent substrate 32. Where the inorganic monocrystalline material having a high index of refraction such as, for example, germanium, silicon, ZnS or ZnSe is employed as a material for the infrared transparent substrate 32, the antireflection film 34 is formed on each of the opposite surfaces of the substrate 32 as shown in FIG. 6A for the purpose of mitigating a surface reflection loss resulting from the difference in refractive index between the air and the substrate 32. Also, at least one of the antireflection films 34 on the substrate 32, particularly the antireflection film 34 interfacing between the ambient air and the optical system of the infrared imaging camera, is coated with the protective film 36 in the form of a diamond film or a diamond-like carbon film for protecting the anti-reflection film 34.
However, both the diamond film and the diamond-like carbon film that are used as the protective film 36 for the infrared transparent optical element are very expensive and result an in increase of the cost of manufacture of the infrared imaging camera.
Accordingly, the present invention has for its primary object to provide an infrared transparent optical element wherein an inexpensive and highly transmissive protective film is employed in place of the expensive diamond film or diamond-like carbon film.
Another object of the present invention is to provide an infrared imaging camera utilizing the infrared transparent optical element of the kind referred to above.
In order to accomplish these and other objects and features of the present invention, there is provided an infrared transparent optical element for use as an optical window for an infrared imaging camera. This infrared transparent substrate having first and second major surfaces opposite to each other, and an antireflection film formed on each of the first and second major surfaces of the substrate. A protective film made of a polymer material is formed on an outer surface of the antireflection film on the first major surface of the substrate.
If the polymer material is employed in the form of an ultrahigh molecular weight, high density polyethylene having a weight-average molecular weight not smaller than 1,600,000, the optical element can be obtained which has an excellent impact strength, an excellent abrasion resistance and an excellent weatherability while exhibiting a satisfactory transmission of infrared rays of light. If this protective film made of the ultrahigh molecular weight, high density polyethylene has a thickness not greater than 0.1 mm, not only can reduction in transmissivity be minimized, but also the protective film, if fusion bonded, can be firmly interlocked with the antireflection film on the first major surface of the substrate.
The use of a metal mount is preferred to enclose the substrate in contact with a peripheral side face thereof.
Preferably, the metal mount has annular end faces opposite to each other, at least one of the annular end faces is surface roughened, and the protective film covering the first major surface of the substrate extends to cover the surface roughened annular end face of the metal mount. In this way, the protective film can be physically interlocked with the annular face of the metal mount while a major portion thereof is fusion bonded to the substrate.
If the metal mount has a coefficient of linear thermal expansion that is greater than that of the substrate, an annular gap can be formed between the substrate and the metal mount when during the manufacture of the optical element the both are heated. This annular gap may be utilized to firmly clamp a portion of the protective film between the substrate and the metal mount upon cooling of the metal mount. To ensure the clamp to be achieved, the difference between the coefficient of linear thermal expansion of the metal mount and that of the substrate is preferably equal to or greater than 2xc3x9710xe2x88x926.
Preferably, the infrared transparent substrate is made of silicon and has a thickness in the range of 1 to 3 mm.
Furthermore, an adhesive layer may be formed between the antireflection film formed on the first major surface of the substrate and the protective film. The adhesive layer is preferably made of polyethylene having a weight-average molecular weight of 10,000 to 500,000. The protective film may include a high-density layer formed on the antireflection film, and a low-density layer formed on the high-density layer. Also, the low density layer may have such a refractive index gradient that the reflection index of the high density layer side is greater than that of the low density layer side.
The infrared transparent substrate is preferred to have a curved surface capable of working as a lens.
According to another aspect of the invention there is provided an infrared imaging camera. This infrared imaging camera includes an infrared transparent optical element according to any one of claims 1 to 13, and an optical system capable of imaging an infrared rays of light which passes through the an infrared transparent optical element.
According to further aspect of the invention there is provided a method of producing an infrared transparent optical element for use as an optical window for an infrared imaging camera including an infrared transparent substrate having first and second major surfaces opposite to each other, wherein infrared rays of light emanating from an object enter the infrared transparent substrate through the first major surface and emerge outwardly from the infrared transparent substrate through the second major surface, an antireflection film formed on each of the first and second major surfaces of the substrate, and a protective film made of a polymer material and formed on an outer surface of the antireflection film on the first major surface of the substrate. This method includes the steps of placing a ultrahigh molecular weight, high density polyethylene film, which is used for the protective film, on the antireflection film of the first major surface so as to form a multi-layer body, and sandwiching the multi-layer body between a pair of heated molds by pressing, and thereby laminating the polyethylene film on the antireflection film of the first major surface side.
Preferably, one of the pair of molds, which is disposed at the polyethylene film side, have a mold surface with a fine roughness, and the low density layer is formed on the protective film by transferring the fine roughness of the mold surface onto the surface of the protective film. The low density layer may be formed on the surface of the protective film by laminating a polyethylene film having a weight-average molecular weight of 10,000 to 500,000 on the protective film and etching the laminated polyethylene film using an acid solution.
It is preferable that at least one of the first and second major surfaces of the infrared transparent substrate has a curved shape, and an elastic member having a mating surface capable of matching with the curved shape is disposed between the mold disposed at the first major surface of the infrared transparent substrate and the multi-layer body. The multi-layer body is sandwiched between the pair of molds through the elastic member and laminated by hot pressing, and thereby fusion bonding the ultrahigh molecular weight, high density polyethylene film to the antireflection film of the first major surface side.
If the curved shape of the infrared transparent substrate is expressed in accordance with a formula by employing the three dimensional xyz co-ordinates
Z=f(x, y), 
the mating surface of the elastic member which is capable of matching with the curved shape may be expressed in accordance with a formula
Z=f(ax, ay)/a, where 0 less than axe2x89xa61. 